In recent years, mobile communication has been used for various purposes, such as voice communication, Internet access, streaming broadcast, distributions of rich contents (e.g. music, video, etc.). In this background, high speed mobile communication is in high demand, and the OFDM (Orthogonal Frequency Division Multiplexing) communication system has attracted public attention as a communication system suitable for high speed data communication.
In general, in an OFDM communication system, the transmission side maps data to be transmitted on each sub-carrier on a frequency axis, and executes an IFFT (Inverse Fast Fourier Transform) process so as to convert the data into a time axis transmission symbol. The reception side executes an FFT (Fast Fourier Transform) for a received signal along its time axis so as to demodulate the received data which has been mapped on the sub-carrier of the frequency region. At this time, an FFT window is set at the reception timing corresponding to the sent symbol timing, to properly demodulate the received data.
Next generation mobile phones and Mobile WiMAX have been considered for use with the OFDM communication system. When the OFDM communication system is used for such applications, a handover process is executed during movement between cells. In the handover process, the frequency of adjacent cells is not necessarily the same. Thus, mobile stations may switch the frequency to another frequency.
Explanations will now be made about a specific example of a handover process during the movement to another cell with a different frequency. FIG. 15 is an example of a wireless communication system. In this example, a mobile station 1 is present in a cell 3 where a frequency A is used and which is formed by a serving base station 2. The mobile station 1 is approaching a cell 5 where a frequency B is used and which is formed by a target base station 4. The serving base station is a radio base station to which the mobile station is connected, while the target base station is a radio base station which is a candidate for the handover process.
In the situation illustrated in FIG. 15, the serving base station 2 sends an instruction for measuring a wireless environment to the mobile station 1 in order to check whether it is appropriate to execute a handover process. For example, the measurement instruction instructs the mobile station 1 to execute a process (scanning process) for measuring the reception quality of a signal transmitted by an adjacent station (target base station). The mobile station 1 executes the scanning process, and reports a measurement result to the serving base station 2.
In this situation, the mobile station 1 executes the scanning process for measuring the reception quality of a signal having the frequency B that is different from the frequency A used for the communication with the serving base station 2. In the following explanations, a process for measuring the reception quality of a signal of a different frequency may be referred to as a “different frequency scanning process”.
FIG. 16 is a diagram regarding a communication state before and after a different frequency scanning process. The mobile station 1 receives a measurement instruction from the serving base station 2 in a normal communication period for performing communication on the frequency A. Upon reception of the measurement instruction, the mobile station 1 switches the frequency to the frequency B in response to the instruction, and measures the reception quality of a signal sent by the target base station 4. Upon completion of the measurement, the mobile station 1 turns the frequency back into the frequency A in order to send the measurement result to the serving base station 2. In the following explanations, in the different frequency scanning process, the period for temporarily switching the frequency for measuring the reception quality of a signal to be sent by the target base station may be referred to as a “scanning period”.
In this manner, conventionally, a synchronizing process is executed when measuring the reception quality of the signal sent by the target base station by temporarily switching the frequency. As described above, in the OFDM communication system, the timing for setting the FFT window needs to be appropriately adjusted in order to appropriately demodulate received data. However, as illustrated in FIG. 17, signals sent from the serving base station 2 and the target base station 4 are received by the mobile station 1 at a different timing based on a difference of transmission timings or a delay difference of transmission paths. Therefore, conventionally, a different frequency scanning process has been executed with a synchronizing process, in accordance with a procedure in FIG. 18.
FIG. 18 is a flowchart regarding a procedure of a conventional different frequency scanning process. As illustrated in FIG. 18, upon request of a different frequency scanning process from the serving base station (Yes in Step S11), the mobile station changes the frequency into the frequency of the target base station (Step S12).
The mobile station executes a frame/symbol synchronizing process in order to appropriately demodulate a signal from the target base station (Step S13). The frame/symbol synchronizing process is disclosed, for example, in patent document 1. The mobile station measures the reception quality of a signal sent by the target base station (Step S14), turns the frequency back into the frequency of the serving base station (Step S15), and sends the measurement result to the serving base station (Step S16).
However, a problem of extending a scanning period exists in a conventional reception quality measurement method for performing a synchronizing process before measuring the reception quality in a different frequency scanning period. The scanning period in the different frequency scanning process is a period in which communication is not able to be performed with the serving base station, that is, a period in which services (e.g., telephone communication) is not offered. However, the time requiring the synchronizing process increases as the movement speed of the mobile station increases.
FIG. 19 is a diagram of a simulation result of the time required for the timing synchronizing process. As illustrated in FIG. 19, when the moving velocity is 3 km/h, the synchronizing success probability is 100% for a period of receiving six frames. When the moving velocity is 60 km/h, a period for receiving 15 frames is needed to achieve a synchronizing success probability of 100%. Accordingly, the time required for the synchronizing process changes in accordance with the moving velocity of the mobile station. Thus, to appropriately execute the synchronizing process regardless of the moving velocity, a long period of time for executing the synchronizing process is set which results in the problem of extending the scanning period.